This invention relates to the fabrication of three-dimensional objects using extrusion-based layered manufacturing techniques. More particularly, the invention relates to a system and method for supplying solid modeling material to a liquifier carried by an extrusion head at a rate that will generate an output flow rate of the liquifier that relates to a variable velocity of the extrusion head as the modeling material is extruded in a predetermined pattern in three dimensions with respect to a base.
Three-dimensional models are used for functions including aesthetic judgments, proofing a mathematical model, forming hard tooling, studying interference and space allocation, and testing functionality. Extrusion-based layered manufacturing machines build up three-dimensional models by extruding solidifiable modeling material from an extrusion head in a predetermined pattern, based upon design data provided from a computer aided design (CAD) system. Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Danforth et al. U.S. Pat. No. 5,738,817, Batchelder et al. U.S. Pat. No. 5,764,521 and Dahlin et al. U.S. Pat. No. 6,022,207, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
A feedstock of either a liquid or solid modeling material is provided to the extrusion head. Where the feedstock of modeling material is in solid form, the extrusion head brings the feedstock to a flowable temperature for deposition. One technique provides the modeling material to the extrusion head in the form of a filament strand.
In the Stratasys FDM(copyright) modeling machines of the current art which employ a filament feed, modeling material is loaded into the machine as a flexible filament wound on a supply reel, such as disclosed in U.S. Pat. No. 5,121,329. A solidifiable material which adheres to the previous layer with an adequate bond upon solidification and which can be supplied as a flexible filament is used as the modeling material. Motor-driven feed rollers advance the strand of the filament into a liquifier carried by an extrusion head. Inside the liquifier, the filament is heated to a flowable temperature. Flowable modeling material is forced out of a nozzle on the far end of the liquifier, and deposited from the liquifier onto a base. The motor-driven feed rollers pushing filament into the liquifier create a xe2x80x9cliquifier pump,xe2x80x9d wherein the filament itself serves as the piston. As the feed rollers advance filament into the liquifier, the force of the incoming filament strand extrudes the flowable material out from the nozzle. The flow rate of the material extruded from the nozzle is a function of the rate at which the filament is advanced to the head. The flow rate is commanded by controlling the speed of advancement of filament into the liquifier. A controller controls movement of the extrusion head in a horizontal (x, y) plane, controls movement of the base in a vertical z-direction, and controls the rate at which the feed rollers advance filament. By controlling these processing variables in synchrony, the modeling material is deposited in xe2x80x9cbeadsxe2x80x9d layer-by-layer along tool paths defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model.
The extruded material delivered by the liquifier pump has a bead of a cross-sectional area that should ideally be controlled to create an accurate model. Usually, a constant bead width is desired. The bead width is related to the flow rate of material out of the pump as well as the extrusion head velocity. The bead width is also affected by the clearance between the extruding nozzle tip and a previously extruded layer (or the base). If the head velocity were to change while the flow rate were to stay constant, the bead width would vary as well.
One type of rapid prototyping system of the prior art drives the motion of the extrusion head at a constant velocity along a tool path comprising a poly-line. A poly-line is a continuous curve of straight-line segments defined by a list of X-Y coordinate pairs at each vertex. The head velocity is preselected so as to accomplish the general goal of moving the extrusion head quickly along the poly-line while minimizing the displacement from the tool path. As a result, the head velocity must be set slow enough that the deviation from the designated tool path will not exceed the maximum allowable following error for the largest deflection along that poly-line. This type of system is therefore unable to take advantage of the higher head velocities that could be employed along some straighter portions of the poly-line, being limited to the lowest maximum velocity available for any portions of the poly-line having larger deflection angles.
Another type of prototyping system of the prior art allows the extrusion head speed to vary in order to increase the throughput of the modeling machine. The extrusion head speeds up along straight portions in the tool path, and slows down where there are deflection angles or vertices. U.S. Pat. No. 6,054,077 describes one such technique for varying the extrusion head speed, which in an exemplary embodiment uses X-Y trajectory profiling that follows the exponential step response of the liquifier pump. The velocity profile of the extrusion head looks like a xe2x80x9cshark tooth,xe2x80x9d while the pump profile follows a step function.
With any extrusion-based prototyping system, a goal is to extrude liquified modeling material at a rate (known as the output flow rate of the liquifier) that is related to the extrusion head velocity, while also maximizing the extrusion head velocity. A system and method that improves the prototyping system""s ability to meet this goal, by controlling the liquifier output flow rate to match a head velocity profile determined according to the shape of the tool path, is the subject of the present invention.
The present invention is an extrusion apparatus employing a method for controlling the output flow rate of a liquifier. The apparatus includes an extrusion head which moves along a predetermined tool path at an extrusion head velocity. The extrusion head carries the liquifier. The liquifier receives a solid element of a modeling material, heats the modeling material, and outputs a flow of the modeling material at an output flow rate. A material advance mechanism is employed to supply the solid element of modeling material to the liquifier at an input rate which controls the output flow rate. In order to control the output flow rate, an extrusion head velocity profile is determined based on the tool path. The input rate of modeling material to the liquifier is then controlled to produce an output flow rate of modeling material from the liquifier that is proportional to a current extrusion head velocity corresponding to the extrusion head velocity profile.